Static information storage and retrieval – Magnetic bubbles – Guide structure
Reexamination Certificate
2002-12-17
2004-12-28
Lee, John D. (Department: 2874)
Static information storage and retrieval
Magnetic bubbles
Guide structure
C385S123000, C385S127000, C430S290000
Reexamination Certificate
active
06836418
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a slant short-period grating used as an optical filter or the like in the field of optical communications and the like.
This application is based on Patent Application No. 2000-183796 filed in Japan, the contents of which are incorporated herein by reference.
BACKGROUND ART
An optical fiber grating is one example of a fiber type of optical filter. There are long-period grating (LPG) and short-period grating (SPG) types of optical fiber grating.
Conventionally, an optical fiber grating is formed by changing the refractive index of a core at a predetermined grating period in the longitudinal direction of the core. Note that the term grating period refers to the period of this refractive index change. An LPG grating period is approximately several hundred &mgr;m.
In LPG, in a grating section in which a change in the refractive index is formed, light of a predetermined wavelength region from the incident light is coupled with a forward clad mode that moves forward in the same direction as the incident light, and transmitted light is obtained from which the light of this wavelength region has been lost.
In contrast to this, in SPG, the grating period is approximately one half to one third the wavelength of the light. Namely, if the operating wavelength is in the vicinity of 1.55 &mgr;m, then a value of, for example, approximately one third of this is set. As a result, the light of a predetermined wavelength region, from among waveguide modes that are propagated along the core of an optical fiber, is reflected and coupled with a reflection mode, and transmitted light is obtained from which this light has been lost.
LPG has the advantage that there are no micro ripples that cause degradation in the signal waveform. The term micro ripples refers to minute fluctuations in the wavelength spectrum of transmitted light when the horizontal axis is the wavelength and the vertical axis is the transmittance. Therefore, LPG enables a smooth characteristic to be obtained in the wavelength spectrum. A further advantage is that reflected light is practically non-existent.
However, LPG has the disadvantage of it being difficult to obtain arbitrary transmission characteristics due to the difficulty of adjusting the transmission characteristics.
In SPG, in addition to the amount of change in the refractive index of the grating portion and the grating period, by employing a chirped grating in which the grating period is changed by being gradually extended or shortened in the longitudinal direction thereof, it is possible to widen the wavelength region of the lost light and to adjust the intensity of the lost light, and it is possible to obtain arbitrary transmission characteristics comparatively freely.
However, in SPG, multiple reflections are generated by the action of reflected light, and as a result, micro ripples are generated in the wavelength spectrum of the transmitted light creating the problem of it not being possible to obtain smooth wavelength spectrum characteristics. There is also the problem that there is a large amount of reflected light.
For these reasons, recently, the freedom of design allowed by SPG has been used to further the development of slant SPG in which it is even more difficult for micro ripples to occur.
FIG. 24
is a side cross-sectional view showing an example of a slant SPG. A description will now be given of the production method for this slant SPG
The symbol
1
in the drawing is a core. An optical fiber is formed by providing a clad
2
having a lower refractive index than the core
1
on the outer periphery of the core
1
.
The core
1
and clad
2
are both formed from quartz glass. A photosensitive dopant that raises the refractive index of quartz glass when light of a specific wavelength is irradiated onto it is doped to the core
1
. Normally, germanium is used as the photosensitive dopant. The refractive index is raised when ultraviolet light of approximately 240 nm is irradiated onto the germanium doped quartz glass.
Accordingly, when light is irradiated at a predetermined grating period in the longitudinal direction of the core
1
from one side surface of the optical fiber by the interposition of a phase mask or the like, the refractive index of that portion of the core
1
receiving the irradiated light is raised so that a grating portion
4
in which a plurality of high refractive index portions
3
,
3
, . . . are arranged at a predetermined grating period is obtained.
The high refractive index portions
3
,
3
, . . . are formed on an inclination so as to cut across the core
1
without being orthogonal to the center axis B of the core
1
. Moreover, a plurality of high refractive index portions
3
,
3
, . . . are arranged parallel to each other in the longitudinal direction of the core
1
. The direction of a line A that is perpendicular to a high refractive index portions
3
is known as the grating direction. Alternatively, this direction is known as the lattice vector direction of the grating portion.
An angle &thgr; between the grating direction A and the center axis B of the core
1
is known as the slant angle. The size of the inclination of the refractive index portions
3
is represented by this angle &thgr;. Note that in normal SPG the grating direction matches the center axis of the core
1
so that the angle &thgr; is zero.
As a result, light that moves along the core
1
in the same direction as that of the incident light and whose waveguide mode is reflected by the grating portion
4
is irradiated onto the clad
2
, and couples with a reverse clad mode that is moving in the opposite direction to the incident light. Namely, because it does not couple with a reflection mode that moves in reverse along the core
1
, it is difficult for multiple reflections to occur. It is therefore possible to reduce the intensity of the micro ripples obtained in the wavelength spectrum.
FIGS. 25A
,
25
B,
26
A, and
26
B show wavelength spectrums for various slant angles.
Because the waveguide mode couples with a plurality of reverse clad modes, in the waveguide spectrum a plurality of loss peaks are aligned adjacent to each other.
If the slant angle is increased from 0 degrees to 2.9 degrees, 4 degrees, and 5.8 degrees, then the coupling with the reflection mode of the waveguide modes is smallest at 4 degrees. When the slant angle is further increased to 5.8 degrees, then the coupling begins to increase again. Namely, periodic characteristics are demonstrated in which couplings with reflection modes repeatedly increase and decrease as the slant angle increases.
The angle at which the couplings with reflection modes first reach the minimum value is known as the reflection suppression angle (4 degrees in this example, as is shown in FIG.
26
A).
In slant SPG if the slant angle is set in the vicinity of the reflection suppression angle, it is possible to reduce the effects of micro ripples.
However, in a slant SPG that uses a typical single mode optical fiber that has a core and clad having a lower refractive index than the core provided on the outer periphery of the core, with the core being formed from germanium doped quartz glass while the clad is formed from pure quartz glass, if the slant angle is set in the vicinity of the reflection suppression angle, the region where the waveguide mode couples with a clad mode is extended, creating the drawback that it is not possible to obtain a steep wavelength spectrum.
FIG. 27
shows an example of a wavelength spectrum of slant SPG transmitted light obtained when, in the core of the above type of typical single mode optical fiber, the slant short period grating portion is formed such that the slant angle is in the vicinity of the reflection suppression angle at a fixed grating period. The loss region (the region of peak loss) reaches as far as 20 nm or more.
Furthermore, in a slant SPG it is possible in some cases to make divisions into main bands, which are wavelength regions where a large loss peak is obtained in the transmitted light wavelength spectrum, and
Inada Tomosada
Kojima Reiko
Nishide Kenji
Okude Satoshi
Sakamoto Akira
Darby & Darby
Fujikura Ltd.
Lee John D.
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